「身につく」 Usage of this word in this sentence !enter image description here From what i can tell, she is saying something along the lines of 'Even if you have the whole cake, it's only 70 grams' but i'm unsure on the usage of here. Unless my interpretation of the context is wrong of course. EDICT (from Nihongodict) 1\. to master (e.g. a skill) 2\. to become accustomed to (e.g. a lifestyle) 3\. to acquire (e.g. a habit) KOTOBANK ― ―

In 99% of the cases, is used as an idiomatic phrase which is described in EDIT. But that's not the case here in this manga.

This seems to mean something along the lines of "(70 g of sugar) becomes part of your body (and add to your weight)". "Gaining 70 g (0.15 lbs) of weight, at most, is not really a matter" is what she implies here. This is not something you have to memorize, but it can be easily inferred by context.